1. Field of the Invention
This invention relates to an apparatus for drying liquid materials as slurries to transform them into powders with the aid of heated spherical bodies, particularly suitble for powdering various kinds of liquid radioactive wastes produced in radioactive material treating plants such as nuclear power stations.
2. Description of the Prior Art
Radioactive waste materials as radioactive waste liquids, resins, sludges and the like produced in radioactive material treating plants such as nuclear power stations are treated or stored in accordance with their characteristic properties. For example, the radioactive waste liquids are enriched in evaporators to obtain enriched waste liquids which are stored as liquids or mixed with cement or asphalt to solidify in drum cans which are then stored in the plants. The waste resins and sludges are stored in slurry tanks or extracted in centrifugal hydro-extractors and then mixed with cement to solidify in drum cans which are also stored in the plants.
However, these methods are low in storing efficiency requiring a great number of storage tanks or drum cans. To avoid this, it has been proposed that after the enriched waste liquids, waste resins and waste sludges are evaporated, dried and powdered in centrifugal membrane drying apparatuses, the powders are formed in pellets or mixed with asphalt or plastic materials to solidify, thereby reducing the volumes of the wastes.
However, these drying apparatuses have the following disadvantages. Scraping blades provided in the drying apparatuses are rotated at high speeds, so that they are susceptible to wear of parts and vibration resulting in failure or trouble, requiring troublesome and expensive inspection and exchange of parts. Moreover, dried powders often stick to the blades and rotating shafts and grow, so that it requires operators to stop the apparatuses with constant intervals and clean the inner parts of the apparatuses with hot water. Accordingly, it is difficult to operate the apparatuses for long periods of time. Furthermore, as the apparatuses employ externally indirect heating system and temperatures of steam as heat source are lower than 200.degree. C., the efficiency of heat transfer is low and the treating capacity of the apparatuses is small so that the radioactive waste liquids prior to being enriched cannot be directly powdered. In case of treating enriched waste liquids mainly consisting of boric acid and sodium hydroxide produced from pressurized water nuclear power stations, particularly, it is required to adjust the mixed ratio of the boric acid and sodium hydroxide in constant narrow ranges before drying and treating the water liquids, because the mixed ratio for powdering is dependent upon drying or treating temperatures in a manner that the range of allowable mixed ratio is the widest at about 350.degree. C. gut as the temperature is lower, the range becomes narrower. It is difficult to powder the enriched waste liquids when the drying temperature is lower than 200.degree. C.